Not sure what you mean by "showing down". If he's just going to showdown with Ax Kx then yeah, call that river, because if he's not capable of valuebetting one pair, either he rivered two pairs or his draw busted. If you just meant you happened to see showdown with him having Ax Kx regardless of how the betting went it's thinner, but players that will just wait two streets to valuebet Ax are rare, so you can probably still call that against that guy provided he's ever capable of betting as a bluff.